The present invention relates to a network system, a transmitting device and a receiving device, which are used for transmitting a plurality of cells forming a broad band cell string generated in, e.g., a broad band rate network by use of a plurality of narrow band rate lines.
In a network using an ATM (Asynchronous Transfer Mode), if there exists no line having a capacity enough to transmit ATM cells (a broad band cell string) of a broad band cell rate, it is required that the broad band cell string be demultiplexed into narrow band cell strings corresponding to a plurality of low-speed lines and thus be transmitted. There is known a network system incorporating a line demultiplexing cell communication function. Herein, the line dividing the cell communicating function is defined as a function of demultiplexing the broad band cell string into a plurality of narrow band cell strings, and transmitting and receiving the respective narrow band cell strings.
As the network system incorporating the line demultiplexing cell communication function, for example, there exists (1) a network system including a band demultiplexer for demultiplexing a broad band cell string received via a high-speed UNI (User Network Interface) such as (1) OC3C etc into low-speed UNIs and thus transmitting these UNIs and a band multiplexer for generating a broad band cell string by multiplexing narrow band cell strings received from the low-speed UNIs, or (2) a network system including a low-speed line broad band device for generating the broad band cell string, allocating each of a plurality of cells forming this broad band cell string to a plurality of low-speed UNIs and thus transmitting the cells and a low-speed line broad band device for restoring the plurality of cells received via the plurality of low-speed UNIs into the broad band cell string and thus transmitting the broad band cell string.
FIG. 25 is a diagram illustrating an example of the network system including a band demultiplexer 5 and a band multiplexer 6. The network system shown in FIG. 25 is a network system in which a low-speed ATM network is connected as a relay network between high-speed ATM networks. Shown therein is such a construction that the data (cells) are transmitted from a left side to a right side on the sheet surface in FIG. 25.
Referring again to FIG. 25, each of the band demultiplexer 5 and the band multiplexer 6 is connected via the low-speed UNI to the low-speed ATM network 7 having a plurality of low-speed lines (three low-speed lines .alpha.1-.alpha.3 in FIG. 25). A customer premises Equipment (CPE) 2a to which data terminal equipments (DTES) 1a, 1b are respectively connected, is connected via the high UNI to the band demultiplexer 5. A CPE 2b to which DTEs 1c, 1d are respectively connected, is connected via the high UNI to the band multiplexer 6.
In the above-described network system, for example, the DTE 1a transmits the data to the DTE 1c, and the DTE 1b transmits the data to the DTE 1d. In this case, cells X1, X2, . . . , Xn stored with the data are transmitted from the DTE a to the DTE 1c defined as a transmitting destination. Cells Y1, Y2, . . . , Yn held with the data are transmitted from the DTE 1b to the DTE id defined as a transmitting destination.
Thereupon, the CPE 2a multiplexes the plurality of cells received from the DTE a, DTE 1b in order of being received, and transmits the cells to the band demultiplexer 5. At this time, the plurality of cells transmitted from the CPE 2a are form one broad band cell string having order of, e.g., the cell X1, the cell X2, the cell Y1, the cell Y2,
The band demultiplexer 5, upon receiving the plurality of cells forming the above broad band cell string from the CPE 2a, cyclically allocates the respective cells to the low-speed lines .alpha.1-.alpha.3 in the order of being received. For instance, the band demultiplexer 5 transmits the cell X1 to the low-speed line .alpha.1, the cell X2 to the low-speed line .alpha.2, the cell Y1 to the low-speed line .alpha.3, and the cell Y2 to the low-speed line .alpha.1. With this transmission, the broad band cell string is demultiplexed into three narrow band cell strings transmitted via the low-speed lines .alpha.1-.alpha.3. The three narrow band cell strings thus formed are transferred to the band multiplexer 6 via the low-speed lines .alpha.1-.alpha.3.
The band multiplexer 6 receives the narrow band cell string via each of the low-speed lines .alpha.1-.alpha.3, and restores the previous broad band cell string by multiplexing these cell strings. That is, the band width multiplexer 6, upon receiving the plurality of cells forming the narrow band cell strings, forms the previous high-speed cell string therefrom and transfers the high-speed cell string to the CPE 2b.
The CPE 2b, when receiving the plurality of cells forming the broad band cell string from the band multiplexer 6, transfers the cells X1-Xn the DTE 1c in the order of being received from the band multiplexer 6 on the basis of connection address data (address data about of the transmitting destination) stored in each cell. Further, the CPE 2b transfers the cells Y1-Yn the DTE 1d in the order of being received from the band multiplexer 6. In this way, the DTE 1c receives the data transmitted from the DTE 1a, and the DTE 1d receives the data transmitted from the DTE 1c.
Incidentally, if a fluctuation of the cells in the low-speed ATM network 7 is 0, and if cell delay times are all the same, the broad band cell string is formed by use of the cells received cyclically by the band multiplexer 6 via the low-speed lines .alpha.1-.alpha.3 in the order of being received, in which case the broad band cell string is restored in the order (the cell X1, the cell X2, the cell Y1, the cell Y2, . . . ) of being transmitted from the band demultiplexer 5.
In reality, however, the cell delay time becomes different according to the low-speed lines .alpha.1-.alpha.3, and the cell fluctuation has a finite value. Therefore, the cells do not arrive at the band multiplexer 6 in the order of being transmitted from the band demultiplexer 5. Accordingly, when the band multiplexer 6 restores the broad band cell string in the order of receiving the cells, the cell order becomes different from the order of being transmitted from the DTE 1a or DTE 1b. In this case, the cells do not arrive at the DTE 1c or DTE 1d defined as a cell receiving equipment in an accurate order. Accordingly, it might happen that an error occurs in the data transmitted to the DTE 1c or DTE 1d.
To cope with this error, the band demultiplexer 5 has a transmitting unit 5a, while the band multiplexer 6 has a receiving unit 6a. Then, the transmitting unit 5a and the receiving unit 6a make the band multiplexer 6 generate the broad band cell string from the plurality of narrow band cell strings in a proper cell order. FIG. 26 is a diagram showing an example of operations of the band demultiplexer 5 (the transmitting unit 5a) and the band multiplexer 6 (the receiving unit 6a) illustrated in FIG. 25.
In order to generate the broad band cell string from the narrow band cell strings transmitted via the plurality of low-speed lines .alpha.1-.alpha.3 in the proper cell order, it is required that a delay in the low-speed ATM network 7 be known beforehand by the band multiplexer 6. Therefore, the transmitting unit 5a of the band demultiplexer 5 transmits training cells TR1-TR3 to the low-speed lines .alpha.1-.alpha.3 at the same timing. Then, the receiving unit 6a of the band multiplexer 6 measures a delay time of each of the low-speed lines .alpha.1-.alpha.3 from a receiving time of each of the training cells TR1-TR2.
The receiving unit 6a does not, however, grasp a transmitting time of each of the training cells TR1-TR3, and therefore measures a relative delay time (T1, T2, . . . Tn: n=3 in the example shown in FIG. 26) on the basis of the receiving time of the training cell received at first among the training cells TR1-TR3.
Subsequently, the receiving unit 6a defines the relative delay time to be Tmax, of the training cell received latest among the training cells TR1-TR3. In the example shown in FIG. 26, the training cell TR2 is received latest, and hence a relative delay time T2 of this training cell TR2 becomes Tmax (T2=Tmax).
Thereafter, the band demultiplexer 5 demultiplexes the cells P1-P8 forming the broad band cell string into three narrow band cell strings (the cell string consisting of the cells P1, P4, P7, the cell string consisting of the cells P2, P5, P8, and the cell string consisting of the cells P3, P6), and transmits these cell strings respectively to the low-speed lines .alpha.1-.alpha.3. The receiving unit 6a, each time the band multiplexer 6 receives the cell via each of the low-speed lines .alpha.1-.alpha.3, adjusts the time by adding a fixed delay time (D1, D2, . . . , Dn) the receiving time of each cell. Herein, the fixed delay time is a value (Di=Tmax-Ti, where i=1 to n) obtained by subtracting the relative delay time relative to each of the training cells TR1-TR3 from the maximum relative delay time Tmax, and is different according to the low-speed line .alpha.1-.alpha.3.
By this time adjustment, the cell order is modified from the receiving order of the band multiplexer 6 to the transmitting order of the band demultiplexer 5. FIG. 26 shows an example in which the cells P1-P3 transmitted in the order of .alpha.1, .alpha.2, .alpha.3 to the low-speed lines .alpha.1-.alpha.3 by the band demultiplexer 5 are received in the order of P1, P3, P2 by the band multiplexer 6 and modified to order of P1, P2, P3 by the receiving unit 6a.
Similarly, the order of the cells P4-P5 is also modified to the order of being transmitted from the band demultiplexer 5. Then, the band multiplexer 6 structures the broad band cell string consisting of the cells P1-P8 in the cell order after being modified, and transfers the same cell string to the CPE 2b.
There arise, however, the following problems inherent in the prior art network system explained above. Namely, in the prior art network system, the delay is adjusted per cell received by the band multiplexer 6. Therefore, before the broad band cell string is transmitted to the low-speed lines .alpha.1-.alpha.3, the band demultiplexer 5 must transmit the training cells TR1-TR2 to the band multiplexer 6. Accordingly, the procedures becomes troublesome.
Further, it might happen that characteristics within the low-speed ATM network 7 change during communications for a long time. In this case, the delay time in each of the low-speed lines .alpha.1-.alpha.3 also changes, and therefore the delay is unable to be properly adjusted in some cases depending on a result of measuring the delay time with respect to the training cells TR1-TR3.
Moreover, if the cell fluctuation within the low-speed ATM network 7 becomes larger than a cell interval in the broad band cell string, the order in the cell string can not be controlled.